1. Field of the Invention
The present invention relates to a resin-molded semiconductor device and a method of producing a lead frame for the same.
2. Description of the Related Art
FIG. 7 shows, in a plan view, a conventional resin-molded semiconductor device. As shown in FIG. 7, the semiconductor device includes a semiconductor element 2a secured to a surface of a die pad is of a lead frame 1 by a soldering layer 4a. Another semiconductor element 2b is secured to a surface of an island 1b of an inner lead 1h of the lead frame by another soldering layer 4b. As best shown in FIG. 8, a cross-sectional view of the device, a resistance wire 3 is secured to the island 1b and an inner lead 1c by soldering layers 4c. Aluminum electrodes (not shown) of the semiconductor elements 2a and 2b are electrically connected with inner leads 1c and 1d by metal wires 5. The above-described components, including the lead frame 1 and the semiconductor elements 2a and 2b, are sealed with a molding resin 6. In FIGS. 7 and 8, the molding resin 6 is indicated only by broken lines.
When the conventional resin-molded semiconductor device having the above-described construction is completed, the magnitude of the current flowing through the resistance wire 3 is controlled by detecting changes in the voltage generated across the resistance wire 3 by the semiconductor element 2b. On the basis of the current flowing through the resistance wire 3, the magnitude of current which can flow through the semiconductor element 2a can be detected.
In this connection, the above-described resin-molded semiconductor device entails the following problem. The resistance wire 3 generates heat due to current flowing therethrough. Therefore, when tests such as operation tests or temperature cycle tests are performed for a long period of time, the soldering layers 4c at the ends of the resistance wire 3 are deteriorated by heat, thereby causing a change in the resistance of the resistance wire 3. In addition, the resistance of the resistance wire 3 may vary in accordance with the amount of the solder in the layers 4c and the amount of climbing of the solder.
The resin-molded semiconductor device has been fabricated by the following method: The semiconductor elements 2a and 2b are soldered onto the front surface of the lead frame 1 in a reduced atmosphere, and the resistance wire 3, already cut into a prescribed length, is simultaneously soldered. Thereafter, the metal thin wires 5 electrically connecting the aluminum electrodes on the semiconductor elements 2a and 2b with the inner leads 1c and 1d are provided by wire bonding. Then, the resultant structure, including the semiconductor elements 2a and 2b already secured and soldered on the lead frame 1 and the island 1b, respectively, is set in a mold in such a manner that the reverse surface of the lead frame 1 is exposed from the mold. In this condition, the entire structure is resin-molded.
The conventional method entails the following problems. Since the resistance wire 3 is joined to the island 1b by soldering, the resistance provided is of poor accuracy, and it also tends to vary, as described before. Since the resistance wire 3 is secured to the island 1b by soldering after the wire 3 has been cut into a prescribed length, the cutting is an additional step in to the entire fabrication process.